During the formation of a semiconductor or flat panel display device, various different process gases are supplied to various process chambers. Chemical vapour deposition (CVD) is used to deposit thin films or layers on the surface of a substrate or wafer located in a deposition chamber. This process operates by supplying one or more reactive gases to the chamber, often using a carrier gas, to the substrate's surface under conditions that encourage chemical reactions to take place at the surface. For example, TEOS and one of oxygen and ozone may be supplied to the deposition chamber for the formation of a silicon oxide layer on the substrate, and silane and ammonia may be supplied for the formation of a silicon nitride layer. Polycrystalline silicon, or polysilicon, is deposited on the substrate by the decomposition of silane or a chlorosilane by heat.
Gases are also supplied to an etch chamber to perform selective etching of areas of the deposited layers, for example during the formation of electrodes and the source and drain regions of a semiconductor device. Etching gases can include the perfluorocompounds such as CF4, CHF3 and NF3, other halocompounds such as HCl, HBr, BCl3, Cl2 and Br2, and combinations thereof. For example, CF4 is commonly used to initially form an opening in a region of a nitride or oxide layer formed over a polysilicon layer and which is exposed by a photoresist layer, and a mixture of HBr and Cl2 is commonly used to subsequently etch the exposed polysilicon.
The etching gases can react with a photoresist to form deposits and tars that need to be periodically removed from the etch chamber, and so a cleaning gas, typically comprising SF6 and oxygen, is periodically supplied to the etch chamber to remove the unwanted material from the chamber.
During these etch and cleaning processes performed within the etch chamber, there is typically a residual amount of the gas supplied to the etch chamber contained in the gas exhaust from the etch chamber. Perfluorocompounds such as CF4 and SF6 are highly dangerous if exhausted to the atmosphere, and so in view of this, before the exhaust gas is vented to the atmosphere, an abatement device is often provided to treat the exhaust gas. The abatement device converts the more hazardous components of the exhaust gas into species that can be readily removed from the exhaust gas, for example by conventional scrubbing, and/or can be safely exhausted to the atmosphere.
Perfluorocompounds (PFCs) such as CF4, SF6, NF3 and C2F6 can be removed from the gas stream with high efficiency using a microwave plasma abatement device. An example of a microwave plasma reactor is described in UK Patent no. GB 2,273,027. In that device, a waveguide conveys microwave radiation from a microwave generator into a gas chamber housing two electrodes in a closely opposed relationship. A gas to be treated flows into the gas chamber through a gas inlet, and passes between the electrodes, so that a microwave plasma is initiated and sustained between the two electrodes from the gas flowing between the electrodes. One of the electrodes has an axial hole to provide a gas outlet from the gas chamber. Under the intensive conditions within the plasma, species within the gas stream are subjected to impact with energetic electrons causing dissociation into reactive species that can combine with oxygen or hydrogen to produce relatively stable by-products.
A convenient source of oxygen and hydrogen for reacting with the various species contained in the gas stream is water vapour, which may be readily added to the gas stream upstream from the abatement device. For example, reaction of CF4 with water vapour will form CO2 and HF, and Cl2 can form HCl within the chamber. HF, HCl and HBr can be subsequently removed from the gas stream by a wet scrubber located downstream from the abatement device.
Various parts of a plasma abatement device, such as one or more of the electrodes, electrode holders and the inner surface of the chamber, are typically formed from stainless steel. In addition to iron, carbon, and chromium, stainless steel may also contain other elements, such as nickel, molybdenum, niobium and titanium. In the presence of air, a passive surface oxide layer is formed, which protects the underlying stainless steel from corrosion. However, in the presence of acidic gases such as HF, HCl, and HBr, the passive layer can be stripped, exposing the underlying stainless steel. For example, HF may be present in the gas chamber as a by-product of the treatment of a perfluorocompound within the gas chamber, and HCl or HBr may be contained in the gas exhaust from the process chamber during an etching process.
Once the nickel oxide layer has been removed, HBr, and HCl can react with the metals contained in the stainless steel when moisture levels are in excess of a few parts per million (ppm). Due to the water adsorbed on the stainless steel surface, the by-products of these reactions can degrade the gas chamber and the electrodes, which can reduce the efficiency of the abatement device.
It is an aim of at least the preferred embodiment of the present invention to provide an improved method of treating a gas stream containing varying amounts of a halocompound, such as HBr, HCl, Br2 and Cl2, and water vapour using a plasma abatement device.